1. Field of the Invention
The present invention relates to an encoder that performs time spreading by converting an optical pulse into chip pulses. Further, the present invention relates to an optical code division multiplexing transmission method that is implemented by using the encoder and a device for implementing this method.
2. Description of Related Art
In recent years, the demand for communications has increased rapidly as a result of the spread of the Internet and so forth. High capacity networks have accordingly been completed at high speed by using optical fiber. Further, in order to establish high-capacity communications, an optical multiplexing technology that transmits a plurality of channels' worth of optical pulse signals together via one optical fiber transmission line has been investigated.
As optical multiplexing technology, optical time division multiplexing (OTDM), wavelength division multiplexing (WDM) and optical code division multiplexing (OCDM) have been intensively researched. Among these technologies, OCDM has the merit of flexibility on the operation side, that is, of having no restrictions on the time axis allocated one bit at a time of the optical pulse signals that are transmitted and received in OTDM and WDM and so forth. Further, OCDM has the merit that a plurality of channels can be established in the same time slot on the time axis or a plurality of communication channels can also be established with the same wavelength on the wavelength axis. Further, a plurality of channels can be multiplexed at the same time at the same wavelength to permit high-capacity data communications. In comparison with OTDM and WDM, and so forth, the focus is on the fact that the communication capacity can increase rapidly (See Hideyuki Sotobayashi, ‘Optical code division multiplexing network’, Applied Physics, Volume 71, 7. (2002) pages 853 to 859, for example).
In the subsequent description, the expression optical pulse signal signifies an optical pulse train reflecting a binary digital signal. That is, an optical pulse train reflecting a binary digital signal in correspondence with the existence and nonexistence of optical pulses constituting the optical pulse train on a time axis with respect to an optical pulse train in which optical pulses stand in a row at regular fixed intervals (time interval corresponding to the reciprocal of the frequency corresponding to the bit rate) is an optical pulse signal.
OCDM is a communication method that extracts signals by means of pattern matching by allocating codes (patterns) that are different for each channel. That is, OCDM is an optical multiplexing technology that encodes an optical pulse signal by means of an optical code that is different for each communication channel on the transmission side and which restores the original optical pulse signal by performing decoding by using the same optical codes on the reception side as on the transmission side. An encoder is used to encode the optical pulse signal to convert same into an encoded optical pulse signal and a decoder is used to decode the encoded optical pulse signal to restore same to an optical pulse signal.
As a method for encoding an optical pulse signal to convert same into an encoded optical pulse signal, a time spreading wavelength hopping method that performs encoding by using both time domains and wavelength domains is known (See Koichi Takiguchi, et al., “Encoder/decoder on planar lightwave circuit for time-spreading/wavelength hopping optical CDMA” OFC 2002, TuK8, March 2002, for example). Further, a phase shift keying (PSK) method that performs encoding by spreading the optical pulse signal over time domains is known (See Naoya Wada, et al., “A 10 Gb/s Optical Code Division Multiplexing Using 8-Chip Optical Bipolar Code and Coherent Detection”, Journal of Lightwave Technology, Vol. 17, No. 10., October 1999 and Akihiko Nishiki, Hideyuki Iwamura, Hisashi Kobayashi, Satoko Kutsuzawa, Saeko Oshiba ‘Development of Encoder/Decoder for OCDM using a SSFBG’ Technical Report of IEICE. OFT2002-66, (2002-11), for example).
In encoding using the PSK method, an example in which an encoder the constituent element of which is a Planar Lightwave Circuit (PLC) is used has been reported (See Naoya Wada, et al., “A 10 Gb/s Optical Code Division Multiplexing Using 8-Chip Optical Bipolar Code and Coherent Detection”, Journal of Lightwave Technology, Vol. 17, No. 10., October 1999). In PSK, a binary phase code (bipolar code) of codelength 8 is used as the code and a transversal-type optical filter is used as the encoding device. The details of the codelength will be described subsequently.
The transversal-type optical filter comprises a delay line, a variable coupling rate optical coupler, a phase modulation section, and a multiplexing section as principle constituent elements (See Koichi Takiguchi, ‘Development of planar light wave circuit into optical function device’ Applied Physics Journal, Volume 72, 11, pages 1387 to 1392 (2003), for example). An optical pulse that is input to a transversal-type optical filter in which there are t variable coupling rate optical couplers is demultiplexed to produce (t+1) optical pulses and the phases of the optical pulses are modulated by the phase modulation section in correspondence with the encoded values. The details will be provided subsequently. However, a plurality of optical pulses generated as a result of the optical pulse input to the encoder being dispersed on the time axis are also known as chip pulses.
The respective variable coupling rate optical couplers are linked by delay lines and the respective chip pulses are multiplexed by the multiplexing section after a delay time has been added by the delay lines to generate a series of chip pulses stream, that is, an encoded optical pulse train.
Further, an example in which a Super Structure Fiber Bragg Grating (SSFBG) is used as the constituent element of the encoder has been reported (See Akihiko Nishiki, Hideyuki Iwamura, Hisashi Kobayashi, Satoko Kutsuzawa, Saeko Oshiba ‘Development of Encoder/Decoder for OCDM using a SSFBG’ Technical Report of IEICE. OFT2002-66, (2002-11)). The SSFBG is constituted by arranging unit Fiber Bragg Gratings (FBG) that are arranged in a row and which correspond one-on-one with code values constituting optical phase code in series in the direction of the optical waveguide. The SSFBG is formed such that unit FBGs in a number equal to the codelength are arranged in a row and established at intervals resulting from the provision of phase shifts that match the code values between the unit FBGs.
As described above, because a passive light element such as an FBG can be used as the phase control means of the encoder, it is possible to deal with higher speeds with respect to the communication rate without the encoding processing being subject to electrical restrictions.
In the subsequent description, suppose that the phase control means used for one channel's worth of encoding is known as an encoder and the device used for a plurality of channels' worth of encoding that integrates a plurality of encoders is known as an encoding device. Further, suppose that the phase control means used for one channel's worth of decoding is known as a decoder and the device for a plurality of channels' worth of decoding that integrates a plurality of decoders is known as a decoding device.
Further, in the subsequent description, an encoder or decoder that is used in the so-called PSK method to perform encoding by spreading an optical pulse signal over time domains is also called an optical pulse time spreader. Further, a device that is constituted by integrating a plurality of optical pulse time spreaders is also called an optical pulse time spreading device.
The operating principles of a case where an optical pulse time spreader that uses an SSFBG is used as an encoder and decoder will now be described with reference to FIGS. 1A to 1E. FIG. 1A shows a time waveform of input optical pulses. FIG. 1E serves to describe an aspect in which an encoded optical pulse train that has been encoded by an encoder is decoded by a decoder.
The input optical pulse shown in FIG. 1A is encoded as a result of being input from an optical fiber 12 to an encoder 10 via an optical circulator 14 and optical fiber 16 as shown in FIG. 1E. The input optical pulse then passes through the optical fiber 18 via the optical fiber 16 and optical circulator 14 once again before being input to a decoder 20 via an optical circulator 22 and optical fiber 24. Further, a cross-correlation waveform is generated as a result of decoding by a decoder 20 and the cross-correlation waveform passes through an optical fiber 26 via the optical fiber 24 and optical circulator 22.
The encoder 10 and decoder 20 shown in FIG. 1E are an SSFBG constituted by arranging four unit FBGs in the waveguide direction of the optical fiber. Here, as an example, the functions of the encoder 10 and decoder 20 will be described by using a four-bit optical code (0, 0, 1, 0). Here, the number of items in the numerical sequence consisting of ‘0’s and ‘1’s that provides the optical code is also called the codelength. In this example, the codelength is 4. Further, the numerical sequence providing the optical code called a code string and each item ‘0’ and ‘1’ of the codelength is also known as a chip. Further, the values 0 and 1 are also called the code values.
The unit FBGs 10a, 10b, 10c, and 10d constituting the encoder 10 correspond with a first chip ‘0’ of the abovementioned optical codes, a second chip ‘0’, a third chip ‘1’, and a fourth chip ‘0’ respectively. The determination of whether the code value is 0 or 1 is the phase relationship of the Bragg reflected light that is reflected by adjacent FBG units.
That is, because the first chip and second chip have an equal code value 0, the phase of the Bragg reflected light reflected by unit FBG 10a corresponding with the first chip and the phase of the Bragg reflected light reflected by unit FBG 10b corresponding with the second chip are equal. Further, because the code value of the second chip is 0 and the code value of the third chip is 1, the two chips have mutually different values. Therefore, the difference between the phase of the Bragg reflected light reflected by unit FBG 10b corresponding with the second chip and the phase of the Bragg reflected light reflected by unit FBG 10c corresponding with the third chip is π.
Likewise, because the code value of the third chip is 1 and the code value of the fourth chip is 0, the two chips have mutually different values. Therefore, the phase of the Bragg reflected light reflected by unit FBG 10c corresponding with the third chip and the phase of the Bragg reflected light reflected by unit FBG 10d corresponding with the fourth chip is π.
Thus, because the phases of the Bragg reflected light from the unit FBGs are changed, the specified optical code is also known as ‘optical phase code’.
A process in which a cross-correlation waveform is formed as a result of an optical pulse being encoded by an encoder and converted to an encoded optical pulse train and the encoded optical pulse train being decoded by a decoder will be described next. When the single optical pulse shown in FIG. 1A is input from the optical fiber 12 to the encoder 10 via the optical circulator 14 and optical fiber 16, Bragg reflected light from the unit FBGs 10a, 10b, 10c, and 10d is generated. Therefore, suppose that the Bragg reflected light from the unit FBGs 10a, 10b, 10c, and 10d is a, b, c, and d. That is, the single optical pulse shown in FIG. 1A is converted into an encoded optical pulse train as a result of time spreading of the Bragg reflected light a, b, c, and d.
When the Bragg reflected light a, b, c, and d is represented on a time axis, an optical pulse train resulting from arrangement at specified intervals that depend on the method of arranging the unit FBGs 10a, 10b, 10c, and 10d on the time axis through division into four optical pulses is constituted as shown in FIG. 1B. Therefore, an encoded optical pulse train is an optical pulse train that is produced as a result of time-spreading an optical pulse that is input to the encoder as a plurality of optical pulses on a time axis. Although the individual optical pulses arranged through time-spreading on the time axis correspond with the respective chip pulses, in cases where there will be no particular confusion in the subsequent description, the chip pulses are also referred to as optical pulses instead of chip pulses.
FIG. 1B shows an encoded optical pulse train that passes through the optical fiber 18 with respect to the time axis. In FIG. 1B, for the purpose of a quick representation of the encoded optical pulse train, the optical pulses are shown displaced in the vertical axis direction.
The Bragg reflected light of unit FBG 10a is the optical pulse denoted by a in FIG. 1B. Likewise, the Bragg reflected light of FBG 10b, FBG 10c, and FBG 10d are optical pulses denoted by b, c, d respectively in FIG. 1B. The optical pulse denoted by a is an optical pulse that is reflected by the unit FBG 10a closest to the input end of the encoder 10 and is therefore in the most temporally advanced position. The optical pulses denoted by b, c, and d are each Bragg reflected light from the FBG 10b, FBG 10c, and FBG 10d respectively. Further, the FBG 10b, FBG 10c, and FBG 10d stand in a line in a row from the input end of the encoder 10 and, therefore, the optical pulses denoted by b, c, and d stand in a line in the order b, c, d after the optical pulse denoted by a as shown by FIG. 1B.
In the subsequent description, the optical pulses corresponding with the Bragg reflected light a, Bragg reflected light b, Bragg reflected light c, and Bragg reflected light d respectively are also represented as the optical pulse a, optical pulse b, optical pulse c, and optical pulse d. Further, the optical pulse a, optical pulse b, optical pulse c, and optical pulse d are also called chip pulses.
The relationship between the phases of the Bragg reflected light a, b, c, and d that constitute the encoded optical pulse train is as follows as mentioned earlier. The phase of the Bragg reflected light a and the phase of the Bragg reflected light b are equal. The difference between the phase of the Bragg reflected light b and the phase of the Bragg reflected light c is π. The difference between the phase of the Bragg reflected light c and the phase of the Bragg reflected light d is π. That is, when the phase of the Bragg reflected light a is taken as the reference, the phases of the Bragg reflected light a, Bragg reflected light b, and Bragg reflected light d are equal and the phase of the Bragg reflected light c differs by π from the phases of the Bragg reflected light a, Bragg reflected light b, and Bragg reflected light d.
Therefore, in FIG. 1B, the optical pulses corresponding with the Bragg reflected light a, the Bragg reflected light b and Bragg reflected light d are denoted by solid lines and the optical pulse corresponding with the Bragg reflected light c is denoted by a dotted line. That is, in order to distinguish the relationship between the phases of the respective Bragg reflected light, solid lines and dotted lines are used to represent the corresponding optical pulses. The phases of the optical pulses denoted by a solid line are in a mutually equal relationship and the phases of optical pulses denoted by dotted lines are in a mutually equal relationship. Further, the phases of the optical pulses denoted by a solid line and the phases of the optical pulses denoted by a dotted line differ by π from one another.
An encoded optical pulse train is input to the decoder 20 via the optical circulator 22 after passing through the optical fiber 18. Although the decoder 20 has the same structure as the encoder 10, the input end and output end are reversed. That is, the unit FBGs 20a, 20b, 20c, and 20d stand in a line in order starting from the input end of the decoder 20 but the unit FBG 20a and unit FBG 10d correspond. Further, a unit FBG 20b, unit FBG 20c and unit FBG 20d likewise correspond with the unit FBG 10c, unit FBG 10b, and unit FBG 10a respectively.
In the encoded optical pulse train that is input to the decoder 20, the optical pulse a constituting the encoded optical pulse train is first Bragg-reflected by the unit FBGs 20a, 20b, 20c, and 20d. This aspect will be described with reference to FIG. 1C. In FIG. 1C, the horizontal axis is the time axis. Further, the relationship before and after a time is illustrated by expediently assigning 1 to 7, where smaller numerical values denote increasingly early times.
FIG. 1C shows an encoded optical pulse train with respect to the time axis in the same way as FIG. 1B. When the encoded optical pulse train is input to the decoder 20, the encoded optical pulse train is first Bragg-reflected by unit FBG20a. The reflected light that is Bragg-reflected by unit FBG20a is shown as ‘Bragg reflected light a’. Likewise, the reflected light that is Bragg-reflected by the unit FBG 20b, unit FBG 20c, and unit FBG 20d is shown as the Bragg reflected light b′, c′, and d′.
The optical pulses a, b, c and d constituting the encoded optical pulse train are Bragg-reflected by unit FBG 20a and stand in a line on the time axis of the string denoted by a′ in FIG. 1C. The optical pulse a that is Bragg-reflected by unit FBG 20a is an optical pulse that has a peak in a certain position that is denoted by 1 on the time axis. The optical pulse b that is Bragg-reflected by unit FBG 20b is an optical pulse with a peak in a certain position that is denoted by 2 on the time axis. Likewise, the optical pulse c and optical pulse d are optical pulses with a peak in a certain position denoted by 3 and 4 respectively on the time axis.
The optical pulses a, b, c, and d that constitute the encoded optical pulse train are also Bragg-reflected by unit FBG 20b and stand in a line on the time axis of the string denoted by b′ in FIG. 1C. The Bragg-reflected reflected light b′ that is reflected by unit FBG 20b has a phase that is shifted by π in comparison with the phases of the Bragg-reflected light a′, c′ and d′. Therefore, the string of optical pulses that stand in a line on the time axis of the string denoted by a′ and the string of optical pulses that stand in a line on the time axis of the string denoted by b′ have phases that are all shifted by π.
As a result, whereas a string of optical pulses that stand in a line in the order 1 to 4 on the time axis denoted by a′ stand in a line in the order of a solid line, solid line, dotted line, and solid line, and a string of optical pulses that stand in a line in the order 2 to 5 on the time axis denoted by b′ stand in a line in the order of a dotted line, dotted line, solid line, and dotted line. The displacement on the time axis of the optical pulse train denoted by a′ and the optical pulse train denoted by b′ is because, among the optical pulses constituting the encoded optical pulse train, the optical pulse a is input to the decoder 20 before the optical pulse b.
Likewise, the optical pulses a, b, c, and d that constitute the encoded optical pulse train are also Bragg-reflected by the unit FBG 20c and unit FBG 20d and the optical pulses stand in a line on the time axis of the strings denoted by c′ and d′ respectively in FIG. 1C. The Bragg-reflected light c′ and d′ reflected by the unit FBG 20c and unit FBG 20d have phases that are equal in comparison with the Bragg-reflected light a′. Therefore, in FIG. 1C, the optical pulse train denoted by c′ and the optical pulse train denoted by d′ stand in a line on the time axis. The optical pulses related to the Bragg-reflected light a′, c′, and d′ are shifted in parallel on the time axis but the mutual phase relationship between the optical pulses related to the Bragg-reflected light is the same.
FIG. 1D shows the cross-correlation waveform of the input optical pulses that are decoded by the decoder 20. The horizontal axis is the time axis and corresponds to the illustration shown in FIG. 1C. The cross-correlation waveform is obtained by the sum of the Bragg-reflected light a′, b′, c′, and d′ from the respective unit FBGs of the decoder and, therefore, all the Bragg-reflected light a′, b′, c′ and d′ shown in FIG. 1C is brought together. Because the optical pulses related to the Bragg-reflected light a′, b′, c′ and d′ are all added together with the same phase at the time shown as 4 on the time axis of FIG. 1C, a maximum peak is formed. Further, because two optical pulses denoted by a dotted line and one optical pulse denoted by a solid line are added together at the times shown as 3 and 5 on the time axis of FIG. 1C, one optical pulse's worth of peaks whose phases differ by π are formed for the maximum peak at the time shown as 4. Further, because two optical pulses denoted by a solid line and one optical pulse denoted by a dotted line are added together at the times shown as 1 and 7 on the time axis of FIG. 1C, one optical pulse's worth of peaks whose phases are equal are formed for the maximum peak at the time shown as 4.
As described hereinabove, the optical pulses are encoded by the encoder 10 to produce an encoded optical pulse train and the encoded optical pulse train is decoded by the decoder 20 to generate a cross-correlation waveform. In the example taken here, an optical code (0,0,1,0) of four bits (codelength 4) is used but the description above is equally valid even in cases where optical code is not used.
The operating principles of a case where the optical pulse time spreader that uses an SSFBG is used as an encoder and decoder were described hereinabove. Here, although a case where the codelength was 4 was taken for the sake of expediency in the description, code with a longer codelength may be used in the actual optical code division multiplexing communication.
In optical code division multiplexing communications, multiplexing is performed by allocating different code to each of the channels. Although distinct codes in a number equal to at least the number of channels are required in order to increase the multiplexed channels, the codelength must be increased in order to increase the number of distinct codes. That is, because one channel is allocated to one code, distinct codes in at least the same number as the number of channels are required.
For example, when an M serial code of codelength 15 is used, two codes are used as the distinct codes. That is, in this case, two-channel optical code division multiplexing communication can be implemented. However, a code of a longer wavelength must be used when there is the desire to implement optical code division multiplexing communication with a greater number of channels. For example, if the codelength is increased to 31, codes of 33 types can be prepared by combining the codes of an M sequence and a Gold sequence. That is, in this case, optical code division multiplexing communication of 33 channels can be implemented.
In order to increase the codelength, either the bit rate of the optical signal must be raised or the spreading time interval must be increased. This fact will be described by using an example in which a case where code of codelength 15 and a case where code of codelength 31 are adopted are compared. The bit rate of the optical signal is related to the subsequently described data rate and chip rate.
When the codelength is 15, if the transmission rate for one channel (also referred to as the ‘data rate’ hereinafter) is 1.25 Gbit/s, the bit rate per chip pulse (also referred to as the ‘chip rate’ hereinafter) is 18.75 Gbit/s (=1.25 Gbit/s×15). That is, the spreading time interval is the reciprocal of the data rate, that is, 5.33×10−7 s (≈(1/18.75)×10−9 s).
On the other hand, when code of codelength 31 is adopted, the chip rate must be made 38.75 Gbit/s (=1.25 Gbit/s×31) in order to equalize the data rate at 1.25 Gbit/s. Further, in order to make the chip rate 18.75 Gbit/s as per the case where code of codelength 15 is used, the data rate must be made 0.605 Gbit/s (≈(1.25 Gbit/s×(15/31). That is, the spreading time interval must be the reciprocal of the data rate, that is, 1.65×10−9 s (≈(1/0.605)×10−9 s).
The method of dealing with a case where the codelength is long may be either that of raising the chip rate with the data rate remaining equal, or lowering the data rate with the chip rate remaining equal, that is, increasing the spreading time interval. In order to raise the chip rate, the transmitter and receiver must be afforded a high speed operation. As a result, the device must be improved and a conversion of the required parts and so forth is required. Such device improvement cannot be implemented easily. Further, the data rate corresponding with code of a long code length must be lowered with the chip rate remaining equal, that is, the spreading time interval must be increased. As a result, the transmission capacity is reduced.